Pointers to compound objects (structs, slices, arrays, maps) are only
followed by fmt if the pointer is at the top level of an argument. This
is to minimise the chances of fmt running into loops.

However, vet did not follow this rule. It likely doesn't help that fmt
does not document that restriction well, which is being tracked in
 #28625.

Updates #27672.

Change-Id: Ie9bbd9b974eda5ab9a285986d207ef92fca4453e
Reviewed-on: https://go-review.googlesource.com/c/147997
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>